Recently, the demand for miniaturized and sophisticated electronic components, for example, power devices or light-emitting diodes (LED) has rapidly increased. As semiconductor elements which can suppress power loss and can highly efficiently achieve power conversion, power devices have become widespread in the fields of electric cars, hybrid cars, quick chargers, etc., and further, the demand therefor is expected to be increased in the new energy fields of photovoltaic power generation systems, mega solar systems, etc.
On the other hand, LED elements having advantages of long service life, small size, and low power consumption as compared with incandescent bulbs have rapidly become widespread in various fields of illumination lamps, cellular phones, liquid crystal panels, automobiles, signal lamps, street lamps, image display devices, etc.
In the advancement of miniaturization and sophistication of electronic components as described above, there is a tendency of increasing the amount of heat generation of semiconductor elements. However, when electronic components are exposed to a high temperature environment for a long period of time, the original function cannot be exhibited, and also the service life is shortened. Therefore, in general, as a joining material for die bonding (a die-bonding material), a joining material having high heat dissipation properties is used for efficiently diffusing heat generated from a semiconductor element. In general, a joining material needs to have a function to efficiently release heat generated from a semiconductor element to a substrate or a housing, and is required to have high heat dissipation properties, although depending on the application thereof.
In this manner, since a joining material to be used for electronic components is required to have high heat dissipation properties, high-temperature lead solder containing much lead or gold-tin solder containing much gold have been widely used. However, high-temperature lead solder has a problem that it contains lead which is considered to be harmful to the human body. Due to this, recently, development of lead-free techniques has been actively performed, and research on conversion to lead-free solder has been actively performed. On the other hand, gold-tin solder contains expensive gold, and therefore has a problem of cost.
In such circumstances, recently, as a potent substitute material substituting for high-temperature lead solder or gold-tin solder, an isotropic electrically conductive adhesive (hereinafter simply referred to as “electrically conductive adhesive”) has attracted attention. The electrically conductive adhesive is a composite of metal particles (for example, silver, nickel, copper, aluminum, or gold) having a function of electrical conductivity and the like, and an organic adhesive (for example, an epoxy resin, a silicone resin, an acrylic resin, or a urethane resin) having an adhesive function, and various metal particles and organic adhesives are used. The electrically conductive adhesive is a liquid at room temperature and therefore is easy to use, and also is free of lead and inexpensive, and thus is a potent substitute material for high-temperature lead solder or gold-tin solder, so that its market is expected to expand greatly.
As described above, the electrically conductive adhesive as the substitute material for solder is required to have high heat dissipation properties as well as electrical conductivity. The organic adhesive which is a raw material of the electrically conductive adhesive basically has a lower thermal conductivity than metals, and therefore, by blending a thermally conductive filler, a heat dissipation function is imparted. The focus of the technical development of the electrically conductive adhesive is how the thermal resistance of the electrically conductive adhesive is decreased to effectively release the generated heat.
Conventionally, as for an electrically conductive adhesive having improved thermal conductivity, for example, Patent Document 1 proposes a highly thermally and electrically conductive composition containing at least 5 to 80 wt % of a pitch-based graphitized carbon fiber filler having an average fiber diameter of 0.1 to 30 μm, an aspect ratio of 2 to 100, an average fiber length of 0.2 to 200 μm, and a true density of 2.0 to 2.5 g/cc, 15 to 90 wt % of a metal fine particle filler having an average particle diameter of 0.001 to 30 μm, and 5 to 50 wt % of a binder resin as solid components in the composition.
Further, Patent Document 2 proposes an electrically conductive composition containing an epoxy resin as a base material resin, a phenol-based curing agent as a curing agent, a urethane-modified epoxy resin as a flexibilizer, and further a powder of gold, silver, copper, iron, aluminum, aluminum nitride, alumina, crystalline silica, or the like as a thermally conductive filler.
Further, Patent Document 3 reports an adhesive containing a resin component, a highly thermally conductive fibrous filler, and a highly thermally conductive spherical filler composed of at least one member selected from the group consisting of silver, gold, platinum, aluminum nitride, silicon oxide, aluminum oxide, and carbon black, wherein with respect to 100 parts by volume of the resin component, the highly thermally conductive fibrous filler is contained in an amount of 0.1 to 20 parts by volume and the highly thermally conductive spherical filler is contained in an amount of to 200 parts by volume.